WiFi calling with our French sim cards works fine but it’s not supported for roaming on our UK (BT Mobile) sim cards. Does anyone here use a UK sim card that supports WiFi calling in France?
It doesn’t really work that way. You just need access to the Internet to make calls with, for example, Whatsapp. That access can be via WiFi, in other words you have logged on to a local WiFi service OR via your SIMs data plan (though you can use both simultaneously). If using WiFi your phone doesn’t even need to have a SIM, just like a tablet or laptop. Does your UK SIM have data roaming?
I took it to mean making VoLTE calls, not using an app such as WhatsApp. Is that what you were referring to, @Vitesse?
I don’t know which UK providers do support VoLTE but I can say that Giffgaff doesn’t support it yet (as I’ve just tested it, and also seen posts on their forum about it not being available - even in the UK).
I haven’t a clue what that is 
What’s used for?
@john_scully I know exactly what I’m talking about. I use WhatsApp all the time. I want to not have to go outside to make and receive voice calls to and from my UK number because we have thick stone walls blocking the signal.
@gareth It’s not VoLTE - that has nothing to do with WiFi - it’s a high quality audio standard that uses 4G mobile data (LTE) to carry the wide band voice data during a call.
Voice over LTE/4G = Voice over IP for cellular devices.
Long story short: LTE/4G didn’t support voice calls at launch, so phones had to switch to 3G or even 2G for voice calls AKA Circuit Switched Fallback (CSFB).
WiFi calling is network providers allowing incoming and outgoing calls using your phone’s voice calling function (not WhatsApp/Telegram/Signal etc) over the WiFi network you’re camped on. You need to have a sniff of cellular service from your home network to register for WiFi calling as your home network needs to figure out your home IP address etc so it can direct calls to you over WiFi instead of paging you via the cellular network.
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Ah, yeah, I was mixing up my VoLTE with my wifi-calling… my bad. In my defence it’s been a few years since I was a Vodaclone… I’ve changed jobs and countries twice since those days 
Back then they used to offer a femtocell that you had to plug-in to your router… think it was called Sure Signal or something like that. Anyway, no sorry, I don’t know of any UK sims that offer wifi-calling internationally.
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Thanks for that. Interesting, it’d passed me by.
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Ahhh, it’s a wall problem, not a technical problem. I’m out 
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So nobody uses a UK sim that supports WiFi calling in France?
I use a Reglo mobile sim in my router for reliable internet and wifi calling. I use Lebara mobile UK sim card in my mobile phone as it has a good usuable whilst roaming data allowance and 100 international minutes included per month. Depending who’s service the Lebara jumps to between usually Free France aerial or Orange France aerial (not so good) I can sometimes make wifi calls indoors on my data plan (using whatsapp or signal ) despite thick walls. I can also use the mobile phone network occasionally.
All a bit weather dependant as I am in a bit of a dip landscape wise, I think the signals skip over us. Last year the aerial mast down the road was being used and it was perfect but it changed to different masts now so not as reliable. The external aerial repeater I bought was helping as well but I havent played with that in a whlle because of more important stuff to be done.
Can you explain what “WiFi calling” is?
@NotALot already did - but in more layman’s terms, if you have a WiFi connection, your call will be routed that way rather than using your mobile signal. This is perfect for when you have a strong WiFi signal but poor Mobile signal. Unfortunately, your mobile network provider needs to support this option. My UK mobile provider, BT Mobile, only supports WiFi calling in the UK, which is not much good for me in France. Why do I have a UK sim while being resident in France? Lots of reasons that are too boring to go into. But I do use WhatsApp (which is linked to my UK mobile number), Facebook Messenger, and Skype voice and video calling, as well as Zoom, and I have a French mobile number too (dual-sim phone). We don’t have a landline (or at least we don’t use one, even though I think it’s provided by default as part of our Orange fibre broadband service.
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Fair enough. Does it have any relevance anymore with the move to 5G?
No, 5G is also a mobile network signal - same issue if your local tower is too far away or blocked by obstacles, like thick stone or concrete walls, etc.
OK, I thought 5G had better propagation. My iPhone has a Free SIM and an Irish eSim that roams and both suffer blackish spots around the house. In which case neither seems to use my meshed WiFi network, which has an excellent signal, everywhere, for voice calls. 
It looks like Lebara supports WiFi calling while roaming? Does anyone here use a Lebara sim card? I think I looked at them some years ago, but there were some deal-breaker drawbacks, but I can’t remember what they were.
I believe Free does support WiFi calling, though I suppose it may be a premium option depending on your contract, but you might also not be getting it because you haven’t enabled it on your phone.